Re: [Qemu-devel] [PATCH for-2.9 14/17] qapi: add static/migration-safe info to query-cpu-model-expansion

[Markus Armbruster <armbru@redhat.com>]

I'm not familiar with CPU model expansion, but here goes anyway.

Eduardo Habkost <ehabkost@redhat.com> writes:

> On x86, "-cpu host" enables some features that can't be
> represented by a static CPU model definition: cache info
> passthrough ("host-cache-info") and PMU passthrough ("pmu"). This
> means a type=static expansion of "host" can't include those
> features.
>
> A type=full expansion of "host", on the other hand, can include
> those features, but then the returned data won't be a static CPU
> model representation.
>
> Add a note to the documentation explaining that when using CPU
> models that include non-migration-safe features, users need to
> choose being precision and safety: a precise expansion of the CPU

s/being/between/

> model (full) won't be safe (static), (because they would include

s/they/it/

> pmu=on and host-cache-info=on), and a safe (static) expansion of
> the CPU model won't be precise.
>
> Architectures where CPU model expansion is always migration-safe
> (e.g. s390x) can simply do what they already do, and set
> 'migration-safe' and 'static' to true.

This patch does that exactly for s390x.  The next patch looks like it
does something for x86.  What about other targets?  I recommend to have
this commit message explain briefly what this patch does, what later
patches in this series do, and what's left for later (if anything).

> Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: libvir-list@redhat.com
> Cc: Jiri Denemark <jdenemar@redhat.com>
> Cc: "Jason J. Herne" <jjherne@linux.vnet.ibm.com>
> Cc: Markus Armbruster <armbru@redhat.com>
> Cc: Eric Blake <eblake@redhat.com>
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
>  qapi-schema.json          | 25 ++++++++++++++++++++++++-
>  target-s390x/cpu_models.c |  4 ++++
>  2 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/qapi-schema.json b/qapi-schema.json
> index 8d113f8..a102534 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -3291,6 +3291,15 @@
>  #        migration-safe, but allows tooling to get an insight and work with
>  #        model details.
>  #
> +# Note: When a non-migration-safe CPU model is expanded in static mode, some
> +# features enabled by the CPU model may be omitted, because they can't be
> +# implemented by a static CPU model definition (e.g. cache info passthrough and
> +# PMU passthrough in x86). If you need an accurate representation of the
> +# features enabled by a non-migration-safe CPU model, use @full. If you need a
> +# static representation that will keep ABI compatibility even when changing QEMU
> +# version or machine-type, use @static (but keep in mind that some features may
> +# be omitted).
> +#
>  # Since: 2.8.0
>  ##
>  { 'enum': 'CpuModelExpansionType',
> @@ -3304,10 +3313,24 @@
>  #
>  # @model: the expanded CpuModelInfo.
>  #
> +# @migration-safe: the expanded CPU model in @model is a migration-safe
> +#                  CPU model. See @CpuDefinitionInfo.migration-safe.
> +#                  If expansion type was @static, this is always true.
> +#                  (since 2.9)
> +#
> +# @static: the expanded CPU model in @model is a static CPU model.
> +#          See @CpuDefinitionInfo.static. If expansion type was @static,
> +#          this is always true.
> +#          (since 2.9)
> +#
> +# query-cpu-model-expansion with static expansion type should always
> +# return a static and migration-safe expansion.
> +#
>  # Since: 2.8.0
>  ##
>  { 'struct': 'CpuModelExpansionInfo',
> -  'data': { 'model': 'CpuModelInfo' } }
> +  'data': { 'model': 'CpuModelInfo', 'static': 'bool',
> +            'migration-safe': 'bool' } }
>  
>  
>  ##
> diff --git a/target-s390x/cpu_models.c b/target-s390x/cpu_models.c
> index 5b66d33..f934add 100644
> --- a/target-s390x/cpu_models.c
> +++ b/target-s390x/cpu_models.c
> @@ -448,6 +448,10 @@ CpuModelExpansionInfo *arch_query_cpu_model_expansion(CpuModelExpansionType type
>      /* convert it back to a static representation */
>      expansion_info = g_malloc0(sizeof(*expansion_info));
>      expansion_info->model = g_malloc0(sizeof(*expansion_info->model));
> +
> +    /* We always expand to a static and migration-safe CpuModelInfo */
> +    expansion_info->q_static = true;
> +    expansion_info->migration_safe = true;
>      cpu_info_from_model(expansion_info->model, &s390_model, delta_changes);
>      return expansion_info;
>  }